High capacity monolithic composite si/carbon fiber electrode architectures synthesized from low cost materials and process technologies

ABSTRACT

A composite Si-carbon fiber comprising a carbon matrix material with 1-90 wt % silicon embedded therein. The composite carbon fibers are incorporated into electrodes for batteries. The battery can be a lithium ion battery. A method of making an electrode incorporating composite Si-carbon fibers is also disclosed.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 13/567,275, filed on Aug. 6, 2012, the entirety of which ishereby incorporated by reference.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with government support under contract No.DE-AC05-000R22725 awarded by the U.S. Department of Energy. Thegovernment has certain rights in this invention.

FIELD OF THE INVENTION

This invention relates generally to batteries, and more particularly toelectrodes for batteries.

BACKGROUND OF THE INVENTION

Silicon has been a sought after anode material for years due to its highcapacity 3800 mAh/g in comparison to battery grade graphite 350 mAh/g.Silicon however does not cycle reversible due to the very significantvolume expansion associated with intercalation and the lack of a stablesolid electrolyte interface (SEI) layer.

Lignin is a waste product of the pulp-and-paper industry produced at arate of millions of tons per year and is burned for calorific value. Itis the principal noncarbohydrate constituent of wood that providesrigidity to the cell walls of plants. A sustainable, renewable resource,lignin is derived from woody plants such as trees and switchgrass.

SUMMARY OF THE INVENTION

A composite Si-carbon fiber comprising a carbon matrix material with1-90 wt % silicon embedded therein. The composite Si-carbon fiber cancomprise 1-30 wt % silicon embedded therein.

The composite Si-carbon fiber can have a diameter of 10-200 μm and alength of at least 3 times the diameter. The composite Si-carbon fibercan have a diameter of 50-100 μm and a length of at least 3 times thediameter.

The silicon can be crystalline with crystals of between 5 nm-200 μm insize. The silicon can be amorphous with particles of between 5 nm-200 μmin size.

The carbon matrix material can be derived from lignin. The carbon matrixmaterial can be derived from an aromatic backbone polymer with a carbonchar yield above 30 wt %. The carbon matrix material can be derived frompolyacrylonitrile. The carbon matrix material can have a density of 2.0g/cm³ or less. The carbon matrix material can have a density of at least1.7 g/cm³.

The silicon is completely embedded inside the composite Si-carbon fiber.

An electrode for a battery includes a composite Si-carbon fibercomprising a carbon matrix material with 1-90 wt % silicon embeddedtherein.

The electrode can have a reversible energy storage capacity of 350 mAh/gto 4000 mAh/g. The electrode can have a reversible energy storagecapacity of 350 mAh/g to 3500 mAh/g.

The electrode can further comprise a binder. The electrode can include aconductive additive. The electrode can include a current collector.

The fibers can be fused together in the absence of a binder. The fibersare fused together and in the absence of a current collector. The fiberscan be provided with a binder and a conductive additive and coated ontoa support. The support can be a current collector.

A battery according to the invention includes an electrode havingcomposite Si-carbon fibers comprising a carbon matrix material with 1-90wt % silicon embedded therein.

A method of making an electrode for a battery includes the step ofproviding carbon matrix material, and mixing silicon particles with thecarbon matrix material to form a carbon-silicon mixture. CompositeSi-carbon fibers are then formed from the carbon-silicon mixture whereinthe silicon is embedded in the fibers. An electrode is formedincorporating the Si-carbon composite fibers into the electrode.

The composite Si-carbon fibers can be mixed with a binder to form theelectrode. The Si-carbon fibers are mixed with carbon black to form theelectrode. The Si-carbon fibers can be mixed with carbon black and abinder to form an electrode. The binder can be a polymeric binder. Thebinder can be Li—C. The Si-carbon fibers are fused together to form amat, with or without a binder.

BRIEF DESCRIPTION OF THE DRAWINGS

There are shown in the drawings embodiments that are presently preferredit being understood that the invention is not limited to thearrangements and instrumentalities shown, wherein:

FIG. 1 is a plot of specific capacity (mAhg⁻¹) vs. rate oflithiation-delithiation (C rate) for a silicon (20 wt %) and lignincomposite Si-carbon fiber, and for a lignin carbon fiber (LCF).

FIG. 2 is a plot of specific capacity (mAhg⁻¹) vs. cycle number atdifferent C rates for Si electrodes having 83% Si-LCF compositeSi-carbon fiber, 13% PVDF, and 4% carbon black, and where the compositeSi-carbon fiber is 20 wt % silicon.

FIG. 3 is a plot of voltage (V) vs. Li/Li⁺ vs. Capacity (mAh).

FIG. 4 is a plot of specific capacity (mAhg⁻¹) vs. cycle number forlithiation and delithiation at differing C-rates for a silicon (20 wt %)and lignin composite Si-carbon fiber.

FIG. 5 is a plot of voltage (V) vs. Li/Li⁺ vs. Capacity (mAh) with theLCF capacity subtracted during Si capacity evaluation.

FIG. 6 is a plot of voltage (V) vs. Li/Li⁺ vs. Capacity (mAh) for bothLCF and Si composite capacity.

FIG. 7 is a plot of specific capacity (mAhg⁻¹) vs. cycle number at C/50rates for Si electrodes having 83% composite Si-carbon fiber, 13% PVDF,and 4% carbon black, and where the composite Si-carbon fiber is 30 wt %silicon.

FIG. 8 is a plot of voltage (V) vs. Li/Li⁺ vs. Capacity (mAh) for bothLCF and Si composite capacity for the electrode having 30 wt % Sicomposite Si-LCF carbon fibers, with both LCF and Si composite capacity.

FIG. 9 is a scanning electron microscopy (SEM) image and a backscattered electron (BSE) image of a section of a composite Si-carbonfiber having 10 wt % silicon.

FIG. 10 is a scanning electron microscopy (SEM) image and a backscattered electron (BSE) image of a section of a composite Si-carbonfiber having 20 wt % silicon.

FIG. 11 is a scanning electron microscopy (SEM) image and a backscattered electron (BSE) image of a section of a composite Si-carbonfiber having 30 wt % silicon.

FIG. 12 is a plot of (a) derivative weight (%/min) and temperaturedifference (° C./mg) vs temperature (° C.) and (b) intensity (arb.units) vs 2θ (deg.)

DETAILED DESCRIPTION OF THE INVENTION

A composite Si-carbon fiber includes a carbon matrix material with 1-90wt % silicon embedded therein. The composite Si-carbon fiber can have awt % range of silicon that is any high value within this range and anylow value within this range. The composite Si-carbon fiber can comprise1-30 wt % silicon embedded therein, or a range of any high value and lowvalue between 1% and 30%.

The composite Si-carbon fiber can have a diameter of 10-200 μm and alength of at least 3 times the diameter. The composite Si-carbon fibercan have a diameter of 50-100 μm and a length of at least 3 times thediameter.

The particle size of the silicon and can vary. The silicon can becrystalline with crystals of between 5 nm-200 μm in size. The siliconcan be amorphous with particles of between 5 nm-200 μm in size. Thesilicon crystals or particles can have a size range with any low valueand any high value within 5 nm-200 μm.

The carbon matrix material is the carbonaceous portion of the compositeSi-carbon fiber. The carbon matrix material of the composite Si-carbonfiber is porous to lithium, which permits the composite carbon fiber tointercalate lithium within the fiber. The lithium enters the compositeSi-carbon fiber by solid state diffusion through defects or grainboundaries that are in the carbon matrix material. Carbon fiberstypically comprise sheets of graphene which are not amenable to thediffusion of lithium. The invention provides a carbon matrix materialwith sufficient grain boundaries and defects that lithium transport intoand out of the composite carbon fibers is possible. The carbon matrixmaterial can have a density of 2.0 g/cm³ or less, which is indicative ofdefects or grain boundaries in the carbon matrix material through whichlithium can be intercalated. The carbon matrix material can have adensity of at least 1.7 g/cm³.

The lithium which is intercalated into the composite Si-carbon fibers istransported to the Si particles that are embedded in the compositeSi-carbon fibers. The swelling of the Si upon such intercalation doesnot damage the fiber as the Si particles are surrounded by the carbonmatrix material. The defects and grain boundaries that provide thediffusion of lithium permit both lithiation and delithiation.

The carbon matrix material can be derived from any suitable carbonsource. In one embodiment, the carbon matrix material is derived fromlignin. In the case of a lignin derived carbon matrix material, defectsor grain boundaries permitting the ingress and egress of lithium arecreated by the removal of aromatic side chains from the lignin. Thecarbon matrix material can be derived from an aromatic backbone polymerwith a carbon char yield above 30 wt %. The carbon matrix material canbe derived from polyacrylonitrile. In the case of non-lignin carbonmatrix materials, the defects can be created by various chemicalprocesses. It is also possible to utilize carbon matrix materials and totreat the carbon matrix materials to create suitable defects or grainboundaries for lithium transport. The amorphous carbon or low carbondensity regions enable facile, isotropic lithium transport throughoutthe fiber structures, providing access to charge storage sites withinthe nanoscale graphitic domains while avoiding high specific surfaceareas associated with nanomaterials.

The silicon can be completely embedded inside the carbon fiber. The term“embedded” as used herein refers to the position of the silicon withinthe surface walls of the composite Si-carbon fibers. In one embodiment,substantially all of the volume of silica in the composite Si-carbonfiber is completely within the carbon matrix material, such that nosilicon is present at the surface of the composite Si-carbon fibers. Inanother aspect, no more than 30%, 25%, 20%, 15%, 10%, 9%, 8%, 7%, 6%,5%, 4%, 3%, 2%, and 1% of the surface area of the composite Si-carbonfiber is silicon, or a range of any high value and low value between 0%and 30%. All of the remainder of the surface area of the compositeSi-carbon fiber can be the carbon matrix material. Surface silicon leftexposed will in some cases be dissolved into the electrolyte such thatlittle or no surface-exposed silicon will be present in the compositeSi-carbon fibers. The silicon is uniformly distributed throughout thebulk of the composite Si-carbon fiber. The silicon at the surface can beproportional to the surface/volume ratio of the composite Si-carbonfiber. The reduced or eliminated silicon exposure to the electrolytewill greatly reduce or even eliminate problems experienced previouslywith silicon electrodes at the electrolyte interface.

An electrode for a battery includes composite Si-carbon fibers accordingto the invention processed by suitable methods to form electrodes. Theelectrodes so formed can have a reversible energy storage capacity of350 mAh/g to 4000 mAh/g. The electrode can have a reversible energystorage capacity of 350 mAh/g to 3500 mAh/g.

The formation of the electrode can include binders to form the compositeSi-carbon fibers into an electrode. Additional additives such asconductive additives can also be included. Such additives includeconductive additives such as carbon black. The composite Si-carbonfiber/binder mixture can then be processed with or without such otheradditives.

The conductivity and mechanical properties of the electrodes can be suchthat a current collector is not necessary. The electrode can include acurrent collector of materials and construction that are known in theart. The fibers can be provided with a binder and a conductive additiveand coated onto a support. The support can be a current collector, or asupportive structure that is not a current collector.

The fibers can be fused together in the absence of a binder. The fiberscan be fused together and in the absence of a current collector. Thecomposite Si-carbon fibers of the invention have good conductivity,chemical stability and mechanical stability.

A battery according to the invention includes an electrode havingcomposite Si-carbon fibers comprising a carbon matrix material with10-90 wt % silicon embedded therein. The battery can be of any suitableconstruction, including lithium ion, lithium air, and others.

Composite Si-carbon fibers according to the invention can be made fromvarious processes. In one embodiment lignin is provided and mixed withsilicon to provide a lignin/silicon mixture. The lignin/silicon mixtureis then extruded into silicon/polymer matrix composite fibers. Thecomposite fiber are thermally processed and converted into carbon fiberscontaining electrochemically active isolated Si particles. Any suitableextrusion or fiber-forming process can be utilized. In one embodiment,the extrusion process is a melt-spinning process. The fiber diameter anddraw ratio are examples of adjustable parameters that control thearchitecture of bundles of individual fibers.

The processing of lignin into electrodes is described in prior pendingapplication Ser. No. 13/567,275 filed Aug. 6, 2012 and entitledLIGNIN-BASED ACTIVE ANODE MATERIALS SYNTHESIZED FROM LOW COST RENEWABLERESOURCES, the disclosure of which is hereby incorporated fully byreference.

The composite Si-carbon fibers are then formed into electrodes. Anysuitable process for forming the electrodes can be utilized. Thecomposite Si-carbon fibers can be mixed with a binder to form theelectrode. The composite Si-carbon fiber and binder can be coated onto asupport or a current collector. The composite Si-carbon fibers can bemixed with conductive additives such as carbon black and no binder, orwith a binder, to form the electrode. The binder can be a polymericbinder. The binder can be Li—C. The composite Si-carbon fibers can beformed into a mat without any binder and fused together to form the mat.The composite Si-carbon fibers can be reacted with oxygen to raise theglass transition temperature to permit carbonization. The fibers arethen carbonized in a manner which does not create SiC.

Thermal processing can define fiber morphology and electrodearchitecture as it can lead to partial melting of lignin at high energysurfaces. The resulting fiber-fiber fusion provides electricalinterconnection throughout the entire electrode mass and electrodedensification. These techniques are extended to Si-in-LCF, and electrodearchitecture. Preservation of rate capabilities is demonstrated throughthe controlled synthesis and electrochemical characterization ofelectrodes. Thorough electron microscopy of the Si/C interface andelectrochemical characterizations it was determined that the interfacialresistance between the two phases is sufficient for good rateperformance. Appreciable diffusion of C into the Si indicating theformation of the electrically inactive phase (SiC) was not detected.

The electrochemical performance of the cathodes comprising Si-LCF as theactive mass was evaluated using two electrode coin-type cells with a 25μm microporous trilayer membrane(polypropylene/polyethylene/polypropylene) separator. Electrodes wereprepared with N-methylpyrrolidone (NMP), slurry of Si-LCF,polyvinylidene fluoride (PVDF), and carbon black (CB) in wt. % ratio of83:13:4. The electrodes comprised ˜2 mg of active Si-LCF per cm² oncopper foil and were about 25 μm thick. Lithium foils (purity 99.9%,Alfa Aesar) were used as counter electrodes. The electrolyte solutionwas 1.2 M LiPF₆ in a 1:2 mixture of ethylene carbonate (EC) and dimethylcarbonate (DMC) by weight (battery grade, Novolyte Technologies, USA).Electrochemical cells were assembled in glove boxes (InnovativeTechnology, Inc., USA) filled with high purity argon. After assembling,the cells were stored at room temperature for about 12 h to ensurecomplete impregnation of the electrodes and separators with theelectrolyte solution. Galvanostatic charge-discharge cycling was carriedout using a multichannel battery tester (model 4000, Maccor Inc., USA)in two-electrode coin-type cells. The cells were charged-dischargedgalvanostatically at rates between C/75 to 10 in the potential rangebetween 0.11/0.1 V-1.2V. At any given C rate each cell is cycled atleast 4 times except for C/75, C/20 and 1C rates. The cells weredischarged at a constant current with a cutoff potential at 0.11/0.1 Vwith a potentiostatic hold for 2 h at 0.11/0.1 V. It should be notedthat electrochemical galvanostatic charge-discharge cycling was carriedout even at lower cut off voltages of 50 mV and 5 mV. However cyclingperformance was poor at low cut off voltages. Galvanostaticcharge-discharge cycling was carried out with at least on three2-electrode coin cells to validate the reproducibility of the results.

FIG. 1 is a plot of specific capacity (mAhg⁻¹) vs. rate oflithiation-delithiation (C rate) for a silicon (20 wt %) and lignincomposite Si-carbon fiber, and for a lignin carbon fiber (LCF). It canbe seen that the electrodes made with composite Si-carbon fibers havesignificantly improved capacity relative to that of the lignin carbonfiber without silicon. The Si-carbon fiber has a very high capacity atlow and medium discharge rates far above state of the art graphite. Itcan also be seen that the lignin carbon fiber has a capacity above stateof the art graphite of less than 300 mAh/g.

FIG. 2 is a plot of specific capacity (mAhg⁻¹) vs. cycle number atdifferent C rates for Si electrodes having 83% composite Si-carbonfiber, 13% PVDF, and 4% carbon black, and where the composite Si-carbonfiber is 20 wt % silicon. It can be seen the Si particles retaingalvanometric capacity at elevated rate electrochemical cycling, andthat the material shows a good cyclability and maintains its highcapacity.

FIG. 3 is a plot of voltage (V) vs. Li/Li⁺ vs. Capacity (mAh). It can beseen that the 1^(st) and subsequent discharge curves are similar. It canalso be seen that there is a minimal voltage hysteresis between thecharge and discharge cycles. The first cycle efficiency loss is on anadequate level consistent with state of the art materials.

FIG. 4 is a plot of specific capacity (mAhg⁻¹) vs. cycle number forlithiation and delithiation at differing C-rates for a silicon (20 wt %)and lignin composite Si-carbon fiber. It can be seen that ratecapability and specific capacity of the material for rates between C/75and 1C is very high.

FIG. 5 is a plot of voltage (V) vs. Li/Li⁺ vs. Capacity (mAh) with theLCF capacity subtracted during Si capacity evaluation. It can be seenthat a considerable amount of Si material is participating in theelectrochemical reaction and the material has a high capacity.

FIG. 6 is a plot of voltage (V) vs. Li/Li⁺ vs. Capacity (mAh) for bothLCF and Si composite capacity. It can be seen that the voltage drop athigher rates is low and resistivity/polarization is adequate.

FIG. 7 is a plot of specific capacity (mAhg⁻¹) vs. cycle number at C/50rates for Si electrodes having 83% Si-LCF composite Si-carbon fiber, 13%PVDF, and 4% carbon black, and where the Si-LCF composite carbon fiberis 30 wt % silicon. It can be seen that the capacity of the materialincreases to fully develop its initial capacity.

FIG. 8 is a plot of voltage (V) vs. Li/Li⁺ vs. Capacity (mAh) for bothLCF and Si composite capacity for the electrode having 30 wt % Sicomposite Si-LCF carbon fibers, with both LCF and Si composite capacity.It can be seen that the capacity of the material increases to fullydevelop its initial capacity.

FIG. 9 is a scanning electron microscopy (SEM) image and a backscattered electron (BSE) image of a section of a composite Si-carbonfiber having 10 wt % silicon. It can be seen that the silicon particlesare fully embedded in the carbon fiber and have a strong interface withthe carbon fiber without exposing silicon surface to the outside of thefiber or environment.

FIG. 10 is a scanning electron microscopy (SEM) image and a backscattered electron (BSE) image of a section of a composite Si-carbonfiber having 20 wt % silicon. It can be seen that the silicon particlesare fully embedded in the carbon fiber and have a strong interface withthe carbon fiber without exposing silicon surface to the outside of thefiber or environment.

FIG. 11 is a scanning electron microscopy (SEM) image and a backscattered electron (BSE) image of a section of a composite Si-carbonfiber having 30 wt % silicon. It can be seen that the silicon particlesare fully embedded in the carbon fiber and have a strong interface withthe carbon fiber without exposing silicon surface to the outside of thefiber or environment.

FIG. 12 is a plot of (a) derivative weight (%/min) and temperaturedifference (° C./mg) vs temperature (° C.) and (b) intensity (arb.units) vs 2θ (deg.) It can be seen that the process window is chosen tofully volatilize any volatile materials and no additional material isbeing removed. The X-ray diffraction shows the crystal structure of thematerial. It can be seen that the silicon is crystalline and the lignincarbon fiber has its typical defect structure. It can also be confirmedthat no silicon carbide or other phases are present.

Example

Si particles were incorporated into the raw lignin to yield compositeSi-carbon fibers consisting of isolated Si particles within asemicontinuous pseudo-graphitic structure. The carbon surfaces of theLCF carbon matrix material will primarily define the SEI, minimizingcomplications of Si SEI chemistry. Lignin is typically extracted as acomplex mix of macromolecules that form a thermoplastic solid at roomtemperature. This material was ground to a powder and mechanicallyblended with Si powder consisting of coarse R<=22 μm particles to yielda mass fraction of 10, 20 or 30 wt. % Si, balance carbon from lignin.Solvent extracted hardwood lignin was selected for this applicationbecause it melts at modest temperatures (slightly above 100° C.) and isstable in air at temperatures above the Tg (glass transitiontemperature) for several hours. These characteristics enable meltprocessing of the mixture into a fibrous morphology biopolymer compositewhile employing only low cost technologies.

The composite fiber is thermally converted into a meso-structural carbonstructure containing the majority of the Si particles embedded within.Scanning electron microscopy (SEM) secondary electrons images (SE) thathighlight fiber topography and back scattered electron images (BSE) thathighlight the contrast between the Si particles and carbon matrix forthe each of the 3 mass fractions are shown in FIGS. 9-11, where the topfigures are SEM images and the bottom figures are BSE images. In generalthe average fiber diameter progressively increased (from 30 μm to over60 μm) with the fraction of Si particles along with increased in surfaceroughness. This was attributed to modifications of rheologicalproperties by addition of non-reacting solid particles that lead to anincreased resistance to melt flow. It was not possible to continuouslyspin fibers containing more than 30 wt. % Si using this technique.

Conversion of the biopolymer into a pseudo-graphitic structure isaccomplished by oxidative stabilization followed by pyrolysis in asweeping inert atmosphere such as flowing nitrogen. In order to minimizeand/or prevent the formation of SiC yet still convert the oxidizedbiopolymer a transient thermal profile was selected and used the lowestheat-treatment temperature possible based on Differential ThermalAnalysis (DTA) and Thermal Galvanometric Analysis (TGA). A selectthermal scan at 10° C./min from ambient to 1400° C. in a nitrogenenvironment, shown in FIG. 12 a, reveals a wide exothermic peak linkedto a decrease in mass. TGA clearly shows 3 distinct regions of pyrolysiswhich is expected from the complex mixture of various molecular weightmolecules found in lignin. Differential Scanning calorimetry (DSC) dataalso indicated a convolution of at least 3 exothermic peaks. Thepyrolysis is complete when the majority of volatiles, oxygen andhydrogen have been driven off as indicated by a flat region in the DTAcurve and cessation of mass loss in TGA. This occurs at approximately900° C. and extends to approximately 1200° C. where indication ofexothermic reactions possibly between Si and C becomes significant.Based on these results 1000° C. was selected as a maximum pyrolysistemperature for this application. BSE images shown in FIGS. 9-11 of thecomposite Si-carbon fiber cross-section reveal a sharp interface betweenthe isolated Si particles and carbon matrix with no discerniblemicro-scale porosity. This indicates that in all cases there was no longrange diffusion of the C into the Si or formation of a microscale layerof SiC. In addition, no evidence of SiC formation was observed in powderx-ray diffraction measurements (FIG. 12 b). The patterns show theexpected broad features corresponding to the LCF and sharp reflectionsfrom the embedded Si powder. No size or strain induced broadening of theSi reflections can be resolved.

The diffraction profiles in FIG. 12 b suggests the matrix material issimilar to LCF anode material produced by this same process withoutsecondary particles included. This was shown to have an internallymodulated mesostructure consisting of high-density, nanoscalecrystalline (graphitic) domains surrounded by a continuous, low-density,highly disordered carbon matrix. Thus, the amorphous carbon network inthe present materials should enable similarly facile, isotropic lithiumtransport throughout the composite Si-carbon fiber structures, providingaccess to charge storage sites within the silicon particles andnanoscale graphitic domains, while supporting the stability of the Simaterial via encapsulation in carbon, and avoiding high specific surfaceareas associated with nanomaterials.

The pseudo-graphitic nanostructure of the characterize lignin carbonfiber as an intercalation material assures the Si particles are alwaysin compression preventing intercalation induced stresses from causingfracture and electrical isolation of the high capacity active material.This mechanism is thought to enable the observed electrochemicalstability in the large embedded Si particles (greater the 20 micron)that otherwise would not cycle reversibly when directly coated onto atypical electrode. Most of the Si particles are never in direct contactwith the electrolyte and this effectively circumvents the chemicallyunstable interface altogether. Alternatively the high capacity particlesreceive Li via solid state diffusion through the pseudo-graphitic phase.The SEI is between the electrolyte/carbon and not between theelectrolyte/Si effectively solving the unstable SEI problem.

The invention provides a next-generation architecture for lithiumbatteries that can potentially circumvent requirements for polymericbinders, conductive aids, and current collectors using low cost,industrially scalable materials and process technologies. Electrodesthat are several hundreds of micrometers thick but maintain ratecapabilities will reduce volume contributions of electrolyte, separator,and tabs. Rate capabilities are maintained by synthesizingthree-dimensional monolithic electrode architectures composed of lignin-

The invention can be embodied in other forms without departing from thespirit or essential attributes there. Accordingly, reference should bemade to the following claims to determine the scope of the invention.

We claim:
 1. A composite Si-carbon fiber comprising a carbon matrixmaterial with 1-90 wt % silicon embedded therein.
 2. The compositeSi-carbon fiber of claim 1, comprising 1-30 wt % silicon embeddedtherein.
 3. The composite Si-carbon fiber of claim 1, wherein the carbonfiber has a diameter of 10-200 μm and a length of at least 3 times thediameter.
 4. The composite Si-carbon fiber of claim 1, wherein thecarbon fiber has a diameter of 50-100 μm and a length of at least 3times the diameter.
 5. The composite Si-carbon fiber of claim 1, whereinthe silicon is crystalline with crystals of between 5 nm-200 μm in size.6. The composite Si-carbon fiber of claim 1, wherein the silicon isamorphous with particles of between 5 nm-200 μm in size.
 7. Thecomposite Si-carbon fiber of claim 1, wherein the carbon matrix materialis derived from lignin.
 8. The composite Si-carbon fiber of claim 1,wherein the carbon matrix material comprises an aromatic backbonepolymer with a carbon char yield above 30 wt %.
 9. The compositeSi-carbon fiber of claim 1, wherein the carbon matrix material isderived from polyacrylonitrile.
 10. The composite Si-carbon fiber ofclaim 1, wherein the carbon matrix material has a density of 2.0 g/cm3or less.
 11. The composite Si-carbon fiber of claim 1, wherein thecarbon matrix material has a density of at least 1.7 g/cm3.
 12. Thecomposite Si-carbon fiber of claim 1, wherein the silicon is completelyembedded inside the carbon fiber.
 13. An electrode for a battery,comprising a composite Si-carbon fiber comprising a carbon matrixmaterial with 1-90 wt % silicon embedded therein.
 14. The electrode ofclaim 13, comprising 1-30 wt % silicon embedded therein.
 15. Theelectrode of claim 13, wherein the composite Si-carbon fiber has adiameter of 10-200 μm and a length of at least 3 times the diameter. 16.The electrode of claim 13, wherein the composite Si-carbon fiber has adiameter of 50-100 μm and a length of at least 3 times the diameter. 17.The electrode of claim 13, wherein the silicon is crystalline withcrystals of between 5 nm-200 μm in size.
 18. The electrode of claim 13,wherein the silicon is amorphous with particles of between 5 nm-200 μmin size.
 19. The electrode of claim 13, wherein the carbon matrixmaterial is derived from lignin.
 20. The electrode of claim 13, whereinthe carbon matrix material comprises an aromatic backbone polymer with acarbon char yield above 30 wt %.
 21. The electrode of claim 13, whereinthe carbon matrix material is derived from polyacrylonitrile.
 22. Theelectrode of claim 13, wherein the carbon matrix material has a densityof 2.0 g/cm3 or less.
 23. The electrode of claim 13, wherein the carbonmatrix material has a density of at least 1.7 g/cm3.
 24. The electrodeof claim 13, wherein the silicon is completely embedded inside thecomposite Si-carbon fiber.
 25. The electrode of claim 13, wherein theelectrode has a reversible energy storage capacity of 350 mAh/g to 4000mAh/g.
 26. The electrode of claim 13, wherein the silicon is uniformlydistributed throughout the bulk of the composite Si-carbon fiber. 27.The electrode of claim 13, wherein the electrode has a reversible energystorage capacity of 350 mAh/g to 3500 mAh/g.
 28. The electrode of claim13, further comprising a binder.
 29. The electrode of claim 13, furthercomprising a conductive additive.
 30. The electrode of claim 13, furthercomprising a current collector.
 31. The electrode of claim 13, whereinthe fibers are fused together in the absence of a binder.
 32. Theelectrode of claim 13, wherein the fibers are fused together and in theabsence of a current collector.
 33. The electrode of claim 13, whereinthe composite Si-carbon fibers are provided with a binder and aconductive additive and coated onto a support.
 34. The electrode ofclaim 33, wherein the support is a current collector.
 35. A batterycomprising an electrode, the electrode comprising composite Si-carbonfibers comprising a carbon matrix material with 1-90 wt % siliconembedded therein.
 36. A method of making an electrode for a battery,comprising the steps of: providing carbon matrix material; mixingsilicon particles with the carbon matrix material to form acarbon-silicon mixture; forming composite Si-carbon fibers from thecarbon-silicon mixture wherein the silicon is embedded in the fibers;and, forming the carbon-silicon composite fibers into an electrode. 37.The method of claim 36, wherein the composite Si-carbon fibers are mixedwith a binder to form the electrode.
 38. The method of claim 36, whereinthe composite Si-carbon fibers are mixed with carbon black to form theelectrode.
 39. The method of claim 36, wherein the composite Si-carbonfibers are mixed with carbon black and a binder to form an electrode.40. The method of claim 39, wherein the binder is a polymeric binder.41. The method of claim 39, wherein the binder is Li—C.
 42. The methodof claim 36, wherein the composite Si-carbon fibers are fused togetherto form a mat.